1. Field of the Invention
The present invention relates generally to a direct-conversion receiver for a mobile communication system. More particularly, the present invention is directed to a direct-conversion receiver for substantially removing DC offset to recover an information signal from a carrier signal modulated by the information signal.
2. Description of the Related Art
Mobile communication systems have evolved from simple one-way (i.e. paging system) communication systems to two-way communication systems comprising analog cellular systems and, more recently, digital cellular systems, e.g., the Global System for Mobile communications (GSM) or Code Division Multiple Access (CDMA). As the size and weight of the mobile communication terminal has been dramatically reduced, terminals that fit comfortably in the palm of one's hand and weighing less than seven ounces are commonplace. Nevertheless, previous receivers using the conventional (super) heterodyne type technology have limitations that restrict their ability to be made smaller and less expensive, because of such constructional elements as external filters, for example an IF SAW filter, and the like.
Therefore, research on alternative techniques to overcome such drawbacks has focused on a new type of receiver in which signals are demodulated and spectrally translated down to the base band directly by having the frequency of the receiver's local oscillator be the same as the frequency of the received carrier signal, which is called a Zero-IF receiver or homodyne receiver. In fact, the concept of direct-conversion had already been suggested by several radio pioneers in the 1920's, and even commercialized in the 1980's for the radio paging receiver. However, there are still several technical problems associated with a direct-conversion type receiver, such as I/Q mismatch, even-order distortion, flicker noise, local oscillator leakage, and DC offset, which make it difficult to replace the current (super) heterodyne type receiver. Fortunately, many of those problems have been solved thus far, with the exception of the DC offset component generated in the direct-conversion receiver.
In general, DC offset is a direct current component that is undesirably generated under several circumstances. The two major causes of the generation of DC offset will be explained with reference to FIGS. 1a and 1b. 
As shown FIG. 1a, since the isolation between local oscillator (LO) 106 and a band pass filter (BPF) 102 or a low noise amplifier (LNA) 103 is not perfect, a strong signal generated in LO 106 exists in BPF 102 or LNA 103, which is called a LO leakage signal. As a result, the LO leakage signal provided to a Quadrature Mixer 104 is multiplied with the LO signal generated in 106, so that both multiplied signals make a direct current component which is called a DC offset.
In the above mentioned case, the LO signal generated in local oscillator 106 and LO leakage signal are represented by the following equation, respectively.LO=ALO×COSwLOt (ALO is the maximum amplitude and wLO corresponds to the carrier frequency fc)  (EQ. 1)LO leakage=Aleak×COS(wLOt+θ), where θ is phase delay caused by LO leakage signal compared to LO signal.  (EQ. 2)
In this case, an output of the Quadrature Mixer 104 is modified as follows:LO×LO leakage=[ALO×COS(wLOt)]×[Aleak×COS(wLOt+θ)]=½ALOAleak×COS(2wLOt+θ)+½ALOAleak×COSθ  (EQ. 3)
Herein, ½ALOAleak×COSθ is represented as a direct current (DC) component which acts like noise after filtering in a low pass filter 105.
FIG. 1b is another example to show how a DC offset component is generated. When a very strong interference (jammer) carrier having the same frequency but different amplitude and phase is provided to the Quadrature Mixer 104′, it also affects a local oscillator 106′ so that both signals coupled in the Mixer 104′ generate a huge direct current (DC) component in the end of intermediate frequency (IF) range.
If the Jammer signal is an interference signal, it is given by the following expression:Ai×COS wit, where Ai is the maximum amplitude and wi corresponds to the interferer frequency fi.  (EQ. 4)
An interference signal, which is coupled into LO input of Quadrature Mixer 104′, is expressed as set forth below:Aileak×(COS wileakt+θ)  (EQ. 5)From the above both expressions, the output of the Quadrature Mixer 104′ is represented by the following equation:[Ai×COS wit]×[Aileak×(COS wileakt+θ)]=½AiAileak×COS(2wit+θ)+½AiAileak×COSθ).  (EQ. 6)
Hereinafter, several conventional technologies to cancel the DC offset component will be introduced.
FIG. 2 illustrates a conceptual architecture of the current direct-conversion receiver. However, the architecture showed in FIG. 2 cannot be used as a receiver in actual situations because it suffers from many problems, including the generation of DC offset component. Therefore, several added circuit components or devices have to be combined with the current direct-conversion receiver of FIG. 2 to avoid those problems. Three examples of the conventional direct-conversion receiver with the additional circuits/components will be explained with FIGS. 3, 4 and 5.
First, FIG. 3 illustrates the structure of a direct-conversion receiver having a capacitor 303 to remove the DC offset component. This kind of architecture has usually been recommended for use with a mobile terminal using the method of time division multiple access (TDMA).
According to FIG. 3, the DC offset component generated by the LO leakage signal is charged in a series capacitor 303 by connecting Switch 304 during the idle time slots, and then the charged DC voltage corresponding to the DC offset component is subtracted during Rx burst.
Next, FIG. 4 illustrates the structure of a direct-conversion receiver including high pass filter (HPF) with a low carrier frequency (fc) to remove the DC offset component. This structure is pertinent to a full-duplex system operating in a broad frequency band. In this case, high pass filter 403 (HPF), which is located between low pass filter 402 (LPF) and low noise amplifier 404 (LNA), can remove not only the DC offset component, but also a DC signal with low frequency. However, if the broad frequency band is used in the system, the damage (i.e., cutting off the low frequencies) caused by HPF 403 would be weaker and weaker, and not affect the capability of the receiver.
Lastly, FIG. 5 illustrates the structure of a direct-conversion receiver using a digital signal processor (DSP) to remove a DC offset component. Referring to FIG. 5, a received signal is transformed into a digital signal in analog-to-digital converter 505 (ADC), and then averaged by digital signal processor (DSP) 507 to obtain a long-term averaging value. In other words, the DC offset component is estimated from the long-term average of the digital signal. The obtained digital value is provided to digital-to-analog converter 509 (DAC) through a memory 508, where the digital value is transformed into analog signal. In adder 503, the analog signal corresponding to the DC offset is subtracted off from the base band signal output from a mixer 502.
However, all three direct-conversion receivers mentioned in the above are only of limited use in a end-use product. That is, the direct-conversion receiver using a capacitor cannot be used with a full-duplex system, e.g. a CDMA communication system. In a TDMA system, interference can appear any time irrespective of the actual signal timing. This could make the receiver ineffective due to external interference. In the case of the direct-conversion receiver using a high pass filter (HPF), not only is the DC offset component removed, but also necessary signal components, such as the SNR (signal noise ratio), may be deteriorated. With the structure shown in FIG. 5, the DC component can be eliminated by averaging the digital signal for a long term in the DSP. Generally, however, this method of averaging a digital signal in real time also experiences problems when the DC offset component is suddenly increased due to the effects of external interference.